A general wireless communication system has considered only one carrier even though bandwidths between an uplink and a downlink are configured to be different from each other. For example, a wireless communication system of which uplink and downlink respectively have a single carrier and uplink bandwidth is generally symmetrical with a downlink bandwidth may be provided on the basis of a single carrier.
In order to obtain a broader bandwidth than that of the existing wireless communication system, the carrier aggregation technology, which groups a plurality of bandwidths in a frequency domain to obtain the same advantage as that obtained by a logically great bandwidth, may be used. In a system that supports carrier aggregation, a cell may be understood by combination of downlink resources and uplink resources. In a wireless communication system which is currently defined, the uplink resources are not required essentially in defining a cell. Accordingly, the cell may be comprised of either the downlink resources only or combination of the downlink resources and the uplink resources. The downlink resources may be referred to as downlink component carriers (DL CC), and the uplink resources may be referred to as uplink component carriers (UL CC). The DL CC and the UL CC may be expressed as carrier frequency that means center frequency in the corresponding cell.
Also, the cell may be classified into a primary cell (PCell) operated on a primary frequency and a secondary cell (SCell) operated on a secondary frequency. The PCell and the SCell may be referred to as serving cells. In brief, the PCell is a core cell related to control in a carrier aggregation condition, and the other serving cells except for the PCell may be the SCells.